1. Field of the Invention
The present invention is in the field of optical detection systems, and, more particularly, in the field of rotation sensors that determine rotation rates by sensing the phase difference between a pair of counterpropagating light waves in an optical loop.
2. Description of the Related Art
Fiber optic rotation sensors typically include a loop of fiber optic material to which light waves are coupled such that a pair of light waves propagate around the loop in opposite directions (i.e., the two light waves are counterpropagating). When the loop is rotated, a relative phase difference is induced between the counterpropagating light waves in accordance with the well-known "Sagnac effect." The amount of phase difference corresponds to the rotational velocity of the loop. The counterpropagating light waves, when recombined, interfere constructively or destructively to produce an optical output signal that varies in intensity in accordance with the rotation rate of the loop. Rotation sensing is commonly accomplished by detection of this optical output signal.
A number of devices and techniques have been developed to process the detected optical output signal to provide an electrical output signal that represents the velocity and direction of rotation of the loop. Known devices and techniques monitor the intensity of the optical output signal to measure the phase difference between the counterpropagating light waves to determine the rotational velocity and direction in accordance with the Sagnac equation: ##EQU1## where: .DELTA..PHI..sub.R is the Sagnac phase difference caused by rotation of the optical loop;
A is the area bounded by the optical loop in which the light waves counterpropagate; PA1 N is the number of times that the light waves propagate around the loop before being recombined; PA1 Q is the angular velocity of the loop about an axis that is perpendicular to the plane of the loop; and PA1 .lambda. and c are the free space values of the wavelength and velocity, respectively, of the light waves applied to the optical loop.
The intensity of the optical output signal is a function of the Sagnac phase difference .DELTA..PHI..sub.R between the two counterpropagating light waves as follows: ##EQU2## where I.sub.T is the intensity of the optical output signal, and I.sub.1 and I.sub.2 are the individual intensities of the two counterpropagating light waves.
It has been found that simple measurement of the intensity of the optical output signal will not provide sufficient information from which the direction and rate of rotation can be determined. For example, the sensitivity of the phase difference measurement is effectively zero for phase differences that are integral multiples of .pi. (i.e., .DELTA..PHI..sub.R =N.pi. for N=. . . -2, -1, 0, 1, 2, . . . ) because the interference intensity is an even, periodic function of the phase difference (i.e., the interference intensity varies as a cosine function as set forth in Equation 2 above. Thus, small phase differences cannot be directly measured near a zero rotation rate. Typically, this difficulty in measuring small phase differences is overcome by dynamic biasing wherein an additional phase modulation is introduced into the closed path around which the light waves are propagating. A time-varying modulation with a zero mean amplitude is generally used rather than a static modulation. The dynamic biasing causes the phase signal to reach values where the sensitivity is suitably large so that the phase difference is readily measurable.
A gyroscope with dynamic biasing can be readily used to measure small rotation rates. When the phase modulation is a harmonic signal of a predetermined frequency, a small phase shift caused by the rotation of the optical loop causes the intensity of the optical output signal to include a time-varying component at the predetermined frequency. This time-varying component can be demodulated to provide a measure of the Sagnac phase shift and thus the rotation rate. Various approaches have been used to demodulate the time-varying component to derive the Sagnac phase shift. Such approaches are described, for example, in U.S. Pat. Nos. 4,410,275; 4,456,377; 4,529,312; 4,634,282; 4,637,722; 4,687,330; 4,707,136; 4,728,192 and 4,779,975.